Polarization inversion, in which the polarization of a single-polarization ferroelectric crystal is partially inverted, enables the control of light waves such as a nonlinear optical effect, an electric optical effect and an acoustic optical effect and is used in a wide variety of fields such as communication, light information processing and measurement. Among them, many studies have made on the application of the polarization inversion to a light wavelength conversion element utilizing the nonlinear optical effect because such application can implement a small-size short wavelength light source in which the wavelength of light from a semiconductor laser is converted.
A conventional method for manufacturing polarization inversion parts comprises forming a comb-shaped electrode and a stripe electrode on the surface of the ferroelectric substrate of an X or Y sheet parallel to a polarization direction and applying a voltage between the electrodes to form polarization inversion parts (Publication of Japanese Patent Application Tokkai Hei No. 4-335620). One conventional method for manufacturing polarization inversion parts will be described with reference to FIG. 58. A first electrode 202 and a second electrode 203 are formed on the ferroelectric LiNbO.sub.3 substrate 201 of an X sheet in the Z direction (the C direction) that is the polarization direction of the LiNbO.sub.3 substrate 201, and the polarization between the electrodes is inverted by applying a voltage between the electrodes from a power supply 205 to periodically form polarization inversion regions 204.
Furthermore, as shown in FIG. 59, another method for forming polarization inversion parts 204 comprises forming a second electrode 203 on a side of a substrate and applying a voltage between a first electrode 202 on a surface of the substrate and the side electrode 203.
Also, a method using a substrate is proposed in which the polarization direction of a crystal is inclined with respect to the surface of the substrate. This method is different from the method shown in FIG. 58 because the spontaneous polarization direction of the substrate is inclined with respect to the crystal surface. Electrodes are formed in the polarization direction of the crystal so that an electric field opposed to the polarization direction of the crystal can be applied. When a voltage is applied between the electrodes, polarization inversion occurs from the first electrode and grows toward the second electrode. Since the polarization inversion grows parallel to the spontaneous polarization of the crystal, the polarization inversion parts get into the substrate along the inclined crystal axis. Therefore, the polarization inversion parts get into the substrate deeper as approaching from the first electrode to the second electrode.
Another conventional method for manufacturing polarization inversion parts is shown in FIG. 60. According to the method shown in FIG. 60, polarization inversion parts are formed in the thickness direction of the substrate.
An electric charge required for polarization inversion is determined by (spontaneous polarization Ps).times.(electrode range area).times.2. The spread of a polarization inversion part is determined by a WIA ratio (A is the period of an electrode and W is the width of the electrode). It is believed that the polarization inversion part spreads only by the value that does not depend on the thickness of the substrate. In the polarization inversion part formed by the method as shown in FIG. 60, the period is about 3 .mu.m, and the area of a region in which the polarization inversion part is formed is about 1 mm.sup.2.
Other examples of the structures of conventional light wavelength conversion elements are shown in FIGS. 61 and 62. The light wavelength conversion element converts the fundamental 208 of light collected in the element to a second harmonic 209 (hereinafter also referred to as SHG) by periodically forming polarization inversion parts 207 on a substrate 206 of a ferroelectric crystal such as LiTaO.sub.3 and performing a phase matching by the polarization inversion parts. Also, in the embodiment shown in FIG. 62, a fundamental 208 is converted to a SHG 209 in an optical waveguide 210 formed on the surface of a substrate 206.
Publication of Japanese Patent Application Tokkai Hei No. 5-273624 discloses a light wavelength conversion element in which a nonlinear deterioration layer is provided near the surface of an optical waveguide. Publication of Japanese Patent Application Tokkai Hei 4-254834 discloses a light wavelength conversion element in which a high refractive index layer having a higher refractive index than an optical waveguide is formed on the optical waveguide. The structural view of this light wavelength conversion element is shown in FIG. 63. In the light wavelength conversion element, a TiO.sub.2 high refractive index layer 211 is formed on the surface of an optical waveguide 210 formed on a LiNbO.sub.3 substrate 206. Since the refractive index of the TiO.sub.2 film 211 is higher than that of the optical waveguide 210, the containment of the fundamental 208 is strong, achieving a more efficient light wavelength conversion element.
Furthermore, another light wavelength conversion element is proposed which uses a ridge-shaped optical waveguide structure to strengthen the containment of an optical waveguide (Publication of Japanese Patent Application Tokkai Hei No. 1-238631).
Problems of the conventional methods for manufacturing polarization inversion parts will be described below.
In a conventional method for manufacturing polarization inversion parts, polarization inversion parts grow from a +C surface, on which polarization inversion cores are formed, to a -C surface. Therefore, a homogeneous polarization inversion structure that has substantially the same shape as an electrode is formed near the +C surface, while the shape of the polarization inversion structure becomes irregular as it approaches the -C surface, forming a polarization inversion structure that does not have a full homogeneity near the -C surface.
Thus, it is difficult to form a light wavelength conversion element that has a deep polarization inversion structure formed by application of an electric field and an optical waveguide formed on a -C surface.
Furthermore, in a conventional method for manufacturing polarization inversion parts using the substrate of an X or Y sheet, it is difficult to form deep polarization inversion parts.
Thus, the depth of polarization inversion parts in a substrate in which the direction of spontaneous polarization is parallel to the surface of the substrate is limited to 1 .mu.m or less. Furthermore, the homogeneity of the polarization inversion parts is not good. Therefore, it is difficult to form polarization inversion parts having a large area, and the interaction length can be only about 10 mm.
In a conventional method for manufacturing polarization inversion parts using a polarization direction and using a substrate in which the polarization direction of a crystal is inclined with respect to the surface of the substrate, polarization inversion parts are formed in the polarization direction of the crystal, so that deep polarization inversion parts are formed. However, the conventional method has the following problems:
1) the effective area that can be used for a light wavelength conversion element is small; and
2) the depth of the polarization inversion parts is limited to about 2 .mu.m, so that it is difficult to form deeper polarization inversion parts.
Polarization inversion parts formed in an oblique substrate get into the substrate from the surface of the substrate. Therefore, only part of the deep polarization inversion parts overlapping an optical waveguide are present near the surface of the substrate. Thus, only part of the polarization inversion parts can be used for an optical waveguide type light wavelength conversion element.
Next, the problems of the conventional light wavelength conversion elements will be described below.
The light wavelength conversion element as shown in FIG. 62 is intended to achieve a high efficiency by forming deep polarization inversion structures using an oblique substrate and enhancing the overlap of the deep polarization inversion structures and an optical waveguide. However, the effective area in which polarization inversion can be used is very narrow and the substrate can have about one optical waveguide overlapping the polarization inversion parts.
The light wavelength conversion element as shown in FIG. 63 is intended to achieve a high efficiency by forming a high refractive index layer 211 having a higher refractive index than a waveguide 210 and strengthening the light containment. However, the high refractive index layer 211 of a ferroelectic film greatly affects the effective refractive index of the waveguide 210, so that a high precision is required for the homogeneity of the thickness of the high refractive index layer 211 on the entire waveguide 210. If the effective refractive index of the waveguide is not controlled well in the propagation direction of light, the conversion efficiency decreases extremely. Therefore, precise homogeneity is required for the thickness of the high refractive index layer 211.
Also, waveguide loss is likely to occur at an interface between the waveguide 210 and the high refractive index layer 211.